Answer by Noel for How do I start/stop mysql server?

On Ubuntu 18.04 the socket is defined in the file /etc/mysql/mysql.conf.d/mysqld.cnf:

 socket     = /var/run/mysqld/mysqld.sock

The directory was not present so I created it manually as below:

sudo mkdir -p /var/run/mysqld

Restart MySQL. That solved it for my case here.